If I was to hazard a guess at the D8 thing I'd go for the D representing the "4", i.e the size designator (at least until they ran out of numbers!), and the 8 representing the model number sequentially, bearing in mind that there have been facelifts of various platforms along the way, hence the 406 being both D8 & D9. Of course that applies logic, which seems to persistently escape french car makers so it might be total bolox.... Anyone know if the 407 is a D10??